1. Field of the Invention
The present invention relates to an optical pickup apparatus and a method for making this apparatus. More specifically, the invention relates to an optical pickup apparatus having a movable unit resiliently supported by springs attached to a fixed unit. The movable unit includes an objective lens for converging an optical beam onto a recording surface of a disk recording medium.
2. Description of the Related Art
An optical disk player reads and writes information on a disk recording medium by moving an optical pickup unit in the radial direction of the rotating disk recording medium. To read out the information recorded on the optical disk, the optical pickup has a focus control function for focusing an optical spot between the information track and the objective lens of the optical pickup unit and a tracking control function for adjusting the position of the optical pickup unit in response to the displacement of the information track in the radial direction of the optical disk.
FIG. 13 illustrates a known optical pickup apparatus having a focusing control function and a tracking control function. In this optical pickup apparatus, an objective lens 1, a focus servo coil 2, tracking servo coils 3, and a bobbin 4 for winding the focus servo coil 2 are mounted on a movable unit 5. The movable unit 5 is resiliently supported by four springs 6 attached to a fixed unit 5. More specifically, the movable unit 5 is resiliently supported by two springs 6 on the left and two springs 6 on the right attached to the fixed units 5. By applying a control current via the springs 6 to the focus servo coil 2 and the tracking servo coils 3, the position of the objective lens 1 is controlled in the focusing and tracking directions.
Japanese Unexamined Patent Application Publication No. 2001-319353 discloses an optical pickup apparatus having an objective lens holder supported by four spring wires so that the holder can be driven in the focusing and tracking directions of the optical disk. A magnetic circuit for driving the holder is displaced so that its center is shifted towards the periphery of the optical disk. The resilience of the spring wires closer to the inner circumference of the optical disk is reduced by increasing the amount of gel, which is a damping material, applied to the spring wires compared to the amount of gel applied to the spring wires closer to the outer circumference of the optical disk. In this way, the holder is driven in the focusing direction at a predetermined angle so that the optical axis of the objective lens is always substantially orthogonal to the signal recording surface of the optical disk, even when the focusing height changes due to warping of the optical disk. Accordingly, the movable unit supporting the objective lens adjusts the skew of the optical pickup apparatus.
In multimedia, large amounts of various information signals, such as image data and audio data, must be processed at once. There are known optical disks having a high recording density and a reduced size that satisfy such a demand. For example, such a disk may have a diameter of 120 mm and a thickness of 1.2 mm, wherein two disks having a disk substrate thickness of 0.6 mm are bonded together, or may have a thickness of 1.2 mm wherein an optical disk having a thickness of 0.6 mm is bonded together with a disk reinforcement plate. Such disks are generally known as a digital versatile disk (DVD). More specifically, a DVD has a reflective surface, i.e., a signal recording surface, formed 0.6 mm inwards in the thickness direction from the signal readout surface.
A known optical disk for increasing the recording density without reducing the allowable limit for tilting the disk is an optical disk having a diameter of 120 mm and a thickness of 1.2 mm formed by bonding an optical disk with a thickness of 0.1 mm with a disk reinforcement plate with a thickness of 1.1 mm. This optical disk is known as a high recording density disk. More specifically, for this high recording density disk, the reflective surface, i.e., the signal recording surface, is formed 0.1 mm inwards from the signal readout surface.
An optical pickup apparatus for reproducing an information signal from a high recording density disk has an objective lens with a numerical aperture (NA) of 0.7 or more. For example, the optical pickup apparatus has a two-lens-group wherein the optical axes of the two lenses are aligned. The two-lens-group unit includes a first lens, which is a front lens facing the high recording density disk and a second lens, which is a rear lens disposed so that its optical axis is aligned with the first lens. A NA of 0.7 or more is achieved by using both the first and second lenses.
There is a demand for optical pickup apparatuses capable of reading different types of disks, such as CDs, CD-Rs, or high recording density disks. The above-mentioned optical pickup apparatus for reading information signals from a high recording density disk cannot read other optical disks, such as CDs or DVDs, because the position of the signal recording surface differs with respect to the thickness of the disk. The difference in the position of the signal recording surface causes spherical aberration and wave aberration, making it impossible to read the optical disk. Means for correcting the wave aberration by a liquid crystal element is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-269611.
To correct the wave aberration and provide an optical pickup apparatus capable of reading optical disks having signal recording surfaces at different positions along the thickness of the disk, the liquid crystal element must be accurately aligned with the objective lens. Thus, it is desirable to mount the liquid crystal element on the movable unit supporting the objective lens.
Mounting the liquid crystal element on the movable unit, however, requires power feeding to the focus servo coil and the tracking servo coils that control the objective lens in the optical-axis direction and the horizontal direction, respectively, and to the electrodes of the liquid crystal element. However, the optical pickup apparatus according to FIG. 13 and Japanese Unexamined Patent Application Publication No. 2001-319353 does not have enough springs for feeding power. Thus, means for feeding power to the liquid crystal element, such as a flexible substrate, must be added to the apparatus. For this reason, the accuracy and the efficiency of the assembly are reduced. There is also a problem in that, when power is fed to the liquid crystal element via a flexible substrate, the responsiveness of the focus servo coil and the tracking servo coils is reduced because of the damping of the flexible substrate.